Human pluripotent stem cells (hPSCs) are cells that can differentiate into a large array of cell types. Stem cells are distinguished from other cell types by two important characteristics. First, they are unspecialized cells capable of renewing themselves through cell division, sometimes after long periods of inactivity. Second, under certain physiologic or experimental conditions, they can be induced to become tissue- or organ-specific cells with special functions. In some organs, such as the gut and bone marrow, stem cells regularly divide to repair and replace worn out or damaged tissues. In other organs, however, such as the pancreas and the heart, stem cells only divide under special conditions.
During embryonic development, stem cells form the tissues of the body from three major cell populations: ectoderm, mesoderm and definitive endoderm. Mesoderm gives rise to blood cells, endothelial cells, cardiac and skeletal muscle, and adipocytes. Definitive endoderm generates liver, pancreas and lung. Ectoderm gives rise to the nervous system, skin and adrenal tissues.
Stem cells have potential in many different areas of health and medical research. Some of the most serious medical conditions, such as cancer and birth defects, are due to problems that occur when cells undergo a transformation. Understanding normal cell development and differentiation mechanisms will allow for a better understanding of these conditions.
Another potential application of stem cells, is making cells and tissues for medical therapies. Today, donated organs and tissues are often used to replace those that are diseased or destroyed. Unfortunately, the number of people needing a transplant far exceeds the number of organs available for transplantation. Stem cells offer the possibility of a renewable source of replacement cells and tissues to treat a myriad of diseases, conditions, and disabilities including Parkinson's disease, amyotrophic lateral sclerosis, spinal cord injury, burns, heart disease, diabetes, and arthritis.
Neurodegenerative diseases are characterized by the progressive loss of neurons or the function of neurons. Common neurodegenerative diseases are Parkinson's, Alzheimer's, and Huntington's disease. For example, Parkinson's disease (PD) is a neurodegenerative disorder caused by a progressive degeneration of midbrain dopamine neurons in the substantia nigra pars compacta. There is currently no cure for PD and treatments such as deep brain stimulation and levodopa can alleviate some of the symptoms but loose efficacy over time. The localized nature of the loss of DA neurons in the substantia nigra offers the opportunity for cell replacement therapy by implanting neuronal cells, either neural stem cells (NSC) or neurons, differentiated from human pluripotent stem cells (hPSC) into the brain of PD patients.